Chalcogen Vacancies as Key Drivers of Distinct Physicochemistry in MoS2, MoSe2, and MoTe2 Transition-Metal Dichalcogenide Monolayers

The physicochemical properties of Mo-bearing transition-metal dichalcogenide (TMD) monolayers are critical to their catalytic potential. However, these properties are generally viewed as similar across monolayers, which limits their versatility and potential for synergy in processes such as the reduction reactions of carbon dioxide (CRR), nitrogen (NRR), and oxygen (ORR). This work investigates the role of chalcogen vacancies in modulating the physicochemistry of MoS2, MoSe2, and MoTe2 through density-functional-theory (DFT) computations, focusing on the adsorption of CO, NO, and NO2. Our findings reveal that chalcogen vacancies not only enhance surface reactivity but also impart unique physicochemical characteristics to each material. These effects stem from intrinsic bonding differences within each TMD, resulting in distinct charge availability around exposed Mo atoms and variations in vacancy size, which shape specific surface interactions. Chalcogen vacancies play a more significant role in defining the unique properties of MoS2, MoSe2, and MoTe2 than the chalcogen atoms themselves. While interaction energy differences between pristine monolayers are minimal (under 0.1 eV), vacancies amplify them to over 1 eV, representing an order-of-magnitude increase. Additionally, varying vacancy sizes among the monolayers influence how species incorporate into vacancies and interact with Mo atoms, further enhancing the differences. This variability unlocks substantial potential of TMD sheets for distinct surface chemistries, transforming them from relatively similar to markedly different as defect density rises. Consequently, our findings underscore the potential of Mo-bearing TMDs to achieve unique catalytic performance through defect engineering, providing valuable insights for tailoring these materials toward specialized applications in photocatalysis and electrocatalysis.